If someone in the US buys an engine off eBay from China, how and who collects the tariff?
The US government collects it from the US buyer, it is basically a tax. How that will be done is still up in the air as far as we know here in Canada.
Again, I can see no way other than eBay doing the math and collecting the tariff. EBay already collects State Sales Taxes for each individual State, and many States have different Sales Tax rates in different parts of the State. Kaliphoney has at least a dozen different Sales Tax Rates depending on where you live in the State and/or what City or County you live in, and eBay successfully does those calculations and charges the buyer the tax rate percentage of purchase price commensurate with where they live!
As an example, I live eight or so miles outside the Bishop City limits. My unincorporated community has a tax rate that is 3/4% less than the City of Bishop. When I used to have a P.O. Box down in town and would sometimes have eBay items sent to that address, I was charged the higher City tax rate by eBay, over the rate I would be charged had it been shipped to my home, even though it was processed through the same Post Office in town.
If eBay can make that distinction, then collecting a universal Tariff for all States, should prove no problem, though it will have to be adjusted depending on the source country.
I'll be very surprised if it is done any other way.